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Date: Fri, 31 Jul 2015 17:38:32 -0400
From: Mike Snitzer <snitzer@...hat.com>
To: Ming Lin <mlin@...nel.org>
Cc: lkml <linux-kernel@...r.kernel.org>,
Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
Kent Overstreet <kent.overstreet@...il.com>,
Dongsu Park <dpark@...teo.net>,
Christoph Hellwig <hch@...radead.org>,
Al Viro <viro@...iv.linux.org.uk>,
Ming Lei <ming.lei@...onical.com>, Neil Brown <neilb@...e.de>,
Alasdair Kergon <agk@...hat.com>, dm-devel@...hat.com,
Lars Ellenberg <drbd-dev@...ts.linbit.com>,
drbd-user@...ts.linbit.com, Jiri Kosina <jkosina@...e.cz>,
Geoff Levand <geoff@...radead.org>, Jim Paris <jim@...n.com>,
Joshua Morris <josh.h.morris@...ibm.com>,
Philip Kelleher <pjk1939@...ux.vnet.ibm.com>,
Minchan Kim <minchan@...nel.org>,
Nitin Gupta <ngupta@...are.org>,
Oleg Drokin <oleg.drokin@...el.com>,
Andreas Dilger <andreas.dilger@...el.com>,
Ming Lin <ming.l@....samsung.com>
Subject: Re: [PATCH v5 01/11] block: make generic_make_request handle
arbitrarily sized bios
On Fri, Jul 31 2015 at 5:19pm -0400,
Ming Lin <mlin@...nel.org> wrote:
> On Fri, Jul 31, 2015 at 12:23 PM, Mike Snitzer <snitzer@...hat.com> wrote:
> > On Mon, Jul 06 2015 at 3:44P -0400,
> > Ming Lin <mlin@...nel.org> wrote:
> >
> >> From: Kent Overstreet <kent.overstreet@...il.com>
> >>
> >> The way the block layer is currently written, it goes to great lengths
> >> to avoid having to split bios; upper layer code (such as bio_add_page())
> >> checks what the underlying device can handle and tries to always create
> >> bios that don't need to be split.
> >>
> >> But this approach becomes unwieldy and eventually breaks down with
> >> stacked devices and devices with dynamic limits, and it adds a lot of
> >> complexity. If the block layer could split bios as needed, we could
> >> eliminate a lot of complexity elsewhere - particularly in stacked
> >> drivers. Code that creates bios can then create whatever size bios are
> >> convenient, and more importantly stacked drivers don't have to deal with
> >> both their own bio size limitations and the limitations of the
> >> (potentially multiple) devices underneath them. In the future this will
> >> let us delete merge_bvec_fn and a bunch of other code.
> >>
> >> We do this by adding calls to blk_queue_split() to the various
> >> make_request functions that need it - a few can already handle arbitrary
> >> size bios. Note that we add the call _after_ any call to
> >> blk_queue_bounce(); this means that blk_queue_split() and
> >> blk_recalc_rq_segments() don't need to be concerned with bouncing
> >> affecting segment merging.
> >>
> >> Some make_request_fn() callbacks were simple enough to audit and verify
> >> they don't need blk_queue_split() calls. The skipped ones are:
> >>
> >> * nfhd_make_request (arch/m68k/emu/nfblock.c)
> >> * axon_ram_make_request (arch/powerpc/sysdev/axonram.c)
> >> * simdisk_make_request (arch/xtensa/platforms/iss/simdisk.c)
> >> * brd_make_request (ramdisk - drivers/block/brd.c)
> >> * mtip_submit_request (drivers/block/mtip32xx/mtip32xx.c)
> >> * loop_make_request
> >> * null_queue_bio
> >> * bcache's make_request fns
> >>
> >> Some others are almost certainly safe to remove now, but will be left
> >> for future patches.
> >>
> >> Cc: Jens Axboe <axboe@...nel.dk>
> >> Cc: Christoph Hellwig <hch@...radead.org>
> >> Cc: Al Viro <viro@...iv.linux.org.uk>
> >> Cc: Ming Lei <ming.lei@...onical.com>
> >> Cc: Neil Brown <neilb@...e.de>
> >> Cc: Alasdair Kergon <agk@...hat.com>
> >> Cc: Mike Snitzer <snitzer@...hat.com>
> >> Cc: dm-devel@...hat.com
> >> Cc: Lars Ellenberg <drbd-dev@...ts.linbit.com>
> >> Cc: drbd-user@...ts.linbit.com
> >> Cc: Jiri Kosina <jkosina@...e.cz>
> >> Cc: Geoff Levand <geoff@...radead.org>
> >> Cc: Jim Paris <jim@...n.com>
> >> Cc: Joshua Morris <josh.h.morris@...ibm.com>
> >> Cc: Philip Kelleher <pjk1939@...ux.vnet.ibm.com>
> >> Cc: Minchan Kim <minchan@...nel.org>
> >> Cc: Nitin Gupta <ngupta@...are.org>
> >> Cc: Oleg Drokin <oleg.drokin@...el.com>
> >> Cc: Andreas Dilger <andreas.dilger@...el.com>
> >> Acked-by: NeilBrown <neilb@...e.de> (for the 'md/md.c' bits)
> >> Signed-off-by: Kent Overstreet <kent.overstreet@...il.com>
> >> [dpark: skip more mq-based drivers, resolve merge conflicts, etc.]
> >> Signed-off-by: Dongsu Park <dpark@...teo.net>
> >> Signed-off-by: Ming Lin <ming.l@....samsung.com>
> > ...
> >> diff --git a/block/blk-merge.c b/block/blk-merge.c
> >> index 30a0d9f..3707f30 100644
> >> --- a/block/blk-merge.c
> >> +++ b/block/blk-merge.c
> >> @@ -9,12 +9,158 @@
> >>
> >> #include "blk.h"
> >>
> >> +static struct bio *blk_bio_discard_split(struct request_queue *q,
> >> + struct bio *bio,
> >> + struct bio_set *bs)
> >> +{
> >> + unsigned int max_discard_sectors, granularity;
> >> + int alignment;
> >> + sector_t tmp;
> >> + unsigned split_sectors;
> >> +
> >> + /* Zero-sector (unknown) and one-sector granularities are the same. */
> >> + granularity = max(q->limits.discard_granularity >> 9, 1U);
> >> +
> >> + max_discard_sectors = min(q->limits.max_discard_sectors, UINT_MAX >> 9);
> >> + max_discard_sectors -= max_discard_sectors % granularity;
> >> +
> >> + if (unlikely(!max_discard_sectors)) {
> >> + /* XXX: warn */
> >> + return NULL;
> >> + }
> >> +
> >> + if (bio_sectors(bio) <= max_discard_sectors)
> >> + return NULL;
> >> +
> >> + split_sectors = max_discard_sectors;
> >> +
> >> + /*
> >> + * If the next starting sector would be misaligned, stop the discard at
> >> + * the previous aligned sector.
> >> + */
> >> + alignment = (q->limits.discard_alignment >> 9) % granularity;
> >> +
> >> + tmp = bio->bi_iter.bi_sector + split_sectors - alignment;
> >> + tmp = sector_div(tmp, granularity);
> >> +
> >> + if (split_sectors > tmp)
> >> + split_sectors -= tmp;
> >> +
> >> + return bio_split(bio, split_sectors, GFP_NOIO, bs);
> >> +}
> >
> > This code to stop the discard at the previous aligned sector could be
> > the reason why I have 2 device-mapper-test-suite tests in the
> > 'thin-provisioning' testsuite failing due to this patchset:
>
> I'm setting up the testsuite to debug.
OK, once setup, to run the 2 tests in question directly you'd do
something like:
dmtest run --suite thin-provisioning -n discard_a_fragmented_device
dmtest run --suite thin-provisioning -n discard_fully_provisioned_device_benchmark
Again, these tests pass without this patchset.
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